feat(router): proper grounded system prompts for chat and narration

Replace the placeholder "You are a routing assistant" one-liner with two
purpose-written prompts: a conversational prompt that frames the router as
correx's operator-facing interface to the event-sourced engine (grounded in
workflow state, no fabrication, concise, steering-aware), and a dedicated
narrator prompt for buildNarrationContext that asks for one or two present-tense
sentences naming the stage and outcome.

The longer protected frame shifted token budgets, so the two L2 eviction tests
now size their budget from the measured protected-frame + per-entry cost instead
of hard-coded magic numbers, making them robust to prompt length.

core/router/src/main/kotlin/com/correx/core/router/RouterContextBuilder.kt

@@ -41,8 +41,30 @@ class DefaultRouterContextBuilder(
 ) : RouterContextBuilder {
 
     companion object {
-        private const val SYSTEM_PROMPT =
-            "You are a routing assistant. Provide guidance based on workflow state and conversation context."
+        private val SYSTEM_PROMPT =
+            """
+            You are correx's router — the conversational interface between the operator and a
+            local, event-sourced workflow engine. You are not the engine: stages, tools, and
+            approvals are run by the kernel and recorded as events. Your job is to help the
+            operator understand and steer what the engine is doing.
+
+            Ground every statement in the workflow state and conversation context you are given.
+            Never invent stages, tool results, file contents, or status you have not been shown;
+            if something is not in context, say you do not have it. Be concise and direct — plain
+            sentences, no filler. When the operator is steering an approval, help them state
+            clearly what they want changed.
+            """.trimIndent()
+
+        private val NARRATION_SYSTEM_PROMPT =
+            """
+            You are correx's router, narrating a running workflow to the operator in their live
+            feed. In one or two short sentences, in your own voice, say what just happened and
+            what comes next. Be concrete and grounded in the workflow state you are given — name
+            the stage, the outcome, and the reason for any failure or pause. Do not use lists,
+            headings, or preamble, and do not invent details you were not given. Speak in the
+            present, as events unfold.
+            """.trimIndent()
+
         private const val RECALLED_MEMORY_PREFIX = "[recalled memory]"
     }
 
@@ -288,8 +310,8 @@ class DefaultRouterContextBuilder(
     ): ContextPack {
         val systemPromptEntry = buildContextEntry(
             sourceType = "systemPrompt",
-            sourceId = "router-system",
-            content = SYSTEM_PROMPT,
+            sourceId = "router-narration-system",
+            content = NARRATION_SYSTEM_PROMPT,
             layer = ContextLayer.L0,
             role = EntryRole.SYSTEM,
         )